Generally speaking, there are three types of conventional filtration systems for cleaning a gas of particulate matter suspended therein. One system comprises a porous matrix placed into the gas stream to intercept, collect and retain pollutant particles which cannot follow the gas through the matrix since their inertia prevents them from changing directions abruptly. Another system involves electrostatic precipitation where intrinsic or induced charges on the pollutant particles are used to collect and hold same on a system of surfaces of the appropriate polarity. Finally, a system known as `scrubbing` involves the discharge of finely dispersed water or liquid chemicals into the polluted gas so as to provide a large surface of contact for the absorption of the pollutants and literally wash them out of the gas phase. All these methods have their specific field of application and are selected according to the specific size distribution of the solid pollutant.
It is appreciated that a filter having a 100% efficiency for prolonged periods is highly desirable in certain situations, but is very difficult, if not impossible, to achieve with presently available filtration techniques. The most stringent requirements concerning the number of pollutant particles have to be met for what is called `clean rooms` in the pharmaceutical and electronics industries. Ultra-high-efficiency filters performing to such standards are those which can remove more than 99.9% of airborne particles with a diameter of 1 to 5 microns but they are expensive since initial and operating costs increase much faster than the efficiency of the filter system. The position taken by environmentalists is that the present standards of air sanitation were established to accomodate the available filtration technology rather than according to the desired level of airborne pollutants. For example, though the highly radiotoxic element plutonium should not be tolerated in the atmosphere, there is no presently available filtering system known to guarantee the elimination in the respired air.
Conventional filters can be guaranteed to provide peak performance during prolonged periods only for rigorous conditions as to particle size distribution, pressure load on the filter and particle influx density. Even when these conditions are optimized, prolonged exposure of porous matrix filter as it is charged will steadily reduce its efficiency, especially for particles of an intermediate size on the filter's performance curve by increasing its flow resistance and decreasing its absorptive power. Evidently, precipitators and scrubbers are not as easily overloaded, but they are also less efficient than matrix filters in the removal of particulate matter from an atmosphere.
Accordingly, it is an object of the present invention to provide a filtering system for removing all of the particulate pollutants from an atmosphere or a gas thereby to provide a particulate-free atmosphere or gas.